Lysosomal storage disorders (LSDs) are a devastating class of diseases which affect the ability of the lysosome-endosome system to properly function. The frequent outcome is sequestration of metabolic intermediates and subsequent cell death. Lysosomal processing of cellular components is particularly essential in the central nervous system, where cells have extremely low turnover rates. Therefore, LSDs such as Tay-Sachs disease are defined in part by severe neurodegeneration, with diagnosed children generally not surviving past the age of five. Tay-Sachs disease is caused by single mutations in the gene HEXA, which causes the enzyme -hexosaminidase A (HexA) to be degraded before it can reach the lysosome. The natural substrate for HexA, GM2 ganglioside, is not properly degraded without lysosome-localized HexA, and the buildup of these metabolites leads to cell death. It is well described that many common HexA mutants never exit the endoplasmic reticulum (ER) due to quality control mechanisms that prevent the enzyme from proceeding in the secretory pathway. ER-associated degradation (ERAD) is a constitutive process that entails the detection of malfolded proteins and their retrotranslocation back to the cytosol, where they are degraded by the proteasome. Interestingly, most HexA mutations disrupt the folding of the enzyme but leave the active site intact. Therefore, altering the quality control capabilities of the cell, either by allowing HexA more chances to fold, or by slowing its rate of degradation, should lead to an increase in proper localization and lysosomal activity of the enzyme. Indeed, studies with various LSDs have shown that even a small percent increase in activity at the lysosome may help prevent disease progression. The goal of this proposal is to study the detection and turnover of mutant HexA by ER factors. My hypothesis is that because of its biochemical properties, HexA will be triaged in a similar pathway used for other soluble glycoproteins, such as the 1- antitrypsin mutant, NHK. Using a combination of overexpression, RNA interference, and pharmacological agents, I will determine the temporal requirements for disposal of mutant HexA by chaperones and degradation machinery. Besides a targeted approach that will examine the roles of BiP, protein disulfide isomerases, and other important quality control factors, a SILAC-based proteomics approach will be used to uncover novel contributions of other ER-resident proteins. Using information about the mechanism of HexA degradation, I will specifically alter ER quality control to allow for increased lysosomal activityof mutant HexA. A deeper understanding of the turnover pathway of mutant HexA will allow for the use of drugs that can cross the blood- brain barrier and specifically alter the quality control factors important in the production of HexA without causing global ER stress or affecting general protein folding and ERAD.